Control system



April 7, 1942- R. D. EvANs m-AL- 2,279,228

CONTROL SYSTEM Filed Jan. 5, 1940 Patented Apr. 7, 1942 2.21am com-nor. srs'rmr noten n. Evans ma charles F. warner. swiuvale, Pa., assignors to Westinghouse Electric Manufacturing Company,

Elst Pittsburgh; Pa.,

a corporation of Pennsylvania Application January s, 1940, serial No. 312,511

8Claims.

Our invention relates. generally, to control systems, and more particularly toa control system for limiting the transient-torques incident to the starting and reversing ofv induction motors.

In many industrial applications of induction motors driving heavy loads and machinery having large inertia, considerable breakage of shafts and couplings has been experienced due to excessive torques produced when starting the motors or reversing the motors by reversing the power phase relation. It has been found that these excessive starting and reversing torques are transient in nature, are often several times the magnitude of the normal motor torque, and are eil'ective over a period of several cycles.

An object of our invention is to provide a control system for induction motors which shall function to limit the transient starting and reversing torques of induction motors.

Another object of the invention is to provide a control system for induction motors which shall function to reduce the ratio of the maximum values of the transient starting and reversing torque to the useful starting and reversing torque of the motor.

A further object of the invention is to provide a control system for induction motors which shall function to limit the starting and reversing torques of the motors and to start and reverse the motors in a minimum period of time commensurate with the limited motor torques.

These and other objects and advantages will be apparent from the following detailed description taken in connection with the accompanying drawing. in which:

Figure 1 is a atic view of van induction motor control system embodying the principal features of our invention; and

Fig. 2 is a graphical illustration of the transient torques previously encolmtered in starting and reversing induction motors.

In practicing our invention, we provide an alternating current generator 2 and circuit breakers 4 and I which may be selectively operated by means of relays 8 and I0 under the control of a selective control switch I2 to connect motors I4 and It to the `generator 2 for rotation in a forward or reverse direction. Starting and reversing the motors I4 and Il under load causes transient torques of excessive values as shown in Fig. 2. Relays I8 and 20 are provided for controlling the iield winding of the generator 2 to provide quick reduction oi' the generator excitation and, therefore, its output potential at the time of reversal of the motor connections and quick buildpotential after the motors Agenerator 2 may be driven by any suitable prime mover (not shown) through the shaft 22. The generator 2 is provided with a ileld winding 28 and may be connected by means of the circuit breakers 4 and 8 and a power bus indicated by the conductors A, B and C to energize the induction motors I4 and I6. The motors I4 and I6 are connected to drive a load such as the runout table rolls 24 and 26 through shafts 28 and 80 and coupling 82, and shafts 84 and 36 and coupling 38, respectively.

The control relays 8 and l0 for the circuit breakers 4 and 8 have front contact elements 40 and 42, respectively, and back contact elements 44 and 46 and 48 and 50, respectively. The front contact element 40 and the back contact element 48 control an energizing circuit for the closing coil of the circuit breaker 4 which extends from the positive terminal of a suitable source of electrical energy through the conductor 52, the winding of the closing coil of the circuit breaker 4, conductor I4, contact element 48, conductor 56. contact element 40, and conductor 58 to the negative terminal of the source of energy.

A similar circuit for the closing coil of the circuit breaker 8 is controlled by the contact elements 42 and 44. This circuit extends from the positive terminal of the source of energy through the conductor 8l, the winding ot the closing coil of the circuit breaker 6, conductor 62, contact element 44, conductor t4, contact element 42, and conductor Il to the negative terminal of the source of energy.

The relays 8 and Il may be selectively energized by the control switch I2 which comprises a contact element 68, movable to selectively engage contact elements 18 and 12. When the contact element 88 is moved to the left-hand position to engage the contact element lll, an energizing circuit is established for the winding of the relay 8 which extends from the positive terminal of the source of electrical energy through the contact elements 68 and l0, conductor 14, the winding of relay 8, and the conductor 58 to the negative terminal of the source of energy. A similar -circuit is established for the relay Il when the contact element 68 is moved to right-hand position into engagement with the contact element 12 which extends from the positive terminal of the source of energy through the contact elements "fand 12, a conductor 1B, the winding oi the relay In. and conductor Bt to the negative terminal of the source of energy.

Heretolore induction motors such as I4 and Il have been started and/or reversed by selectively actuating circuit breakers such as 4 and i through relays l and I0 by such a switch as the selector switchil Where such starting and reversal of the motors has been effected with Iull output potential of the generator 2, excessive transient torques occur which severely strain the shafts and couplings between the motors and the driven load and often damage the shafts and couplings. The graph of Fig. 2 illustrates the nature or these transient torques. The graph shows one curve illustrating the transient torque and another curve illustrating the normal starting torque which occurs when full starting potential is applied to a locked rotor induction motor. simulatingthe conditions which are obtained when an induction motor is required to start or reverse a very heavy load. The transient torque reaches magnitudes of several times the normal locked rotor torque of the motor. Since the normal locked rotor starting torque is several'times the normal operating torque of the motor, the transient starting torque isa very considerable multiple oi the normal motor torque. That part of the applicants control system which will be described in detail hereinafter is provided for limiting these transient torques while still providing a minimum oi.' starting and/or reversing period for the motors M and i8.

The control relay 20 is connected to be energized in a circuit which extends from the positive terminal oi.' the source of energy through the contact elements BB and 10, conductor 18, contact element 50, conductors 80 and B2, the winding oi the relay 2l, and the conductor 8l to the negative terminal of the source of energ'y. The winding oi the relay 20 may also be energized in a circuit which extends from the positive terminal of the source of energy through the contact elements lt and 12, a conductor 86, contact element 46, conductors 8U and 82, to the winding of the relay 20 and conductor B4 to the negative terminal of the source of energy. The

relays l and Ill are slow-to-release relays so constructed as to close their front contacts and open their hack contacts immediately upon energization of the relay windings but to delay the opening of the front contacts and the closure o! the back contacts for a short zation oi' their windings.

The front contact 88 oi the control relay 2|) is disposed to connect the field windings 23 of the generator 2 in an. energizing circuit which extends Irom the positive terminal of a source oi direct current energy through a conductor 90, the windings i3, conductors s 93, a resistor 94, conductor tt, the contact sement ad and a conductor 98 to the negative terminal of the source of energy.

The back contact element im ol the relay llt is disposed to control a discharge circuit for the eld windings 'i3 which extends from one side of the field windings 23! 90 and IUI, a resistor ini, conductor W4, contact element Idil, and a conductor litt to the other side of the field windings 2t.

The field windings 23 of the generator 2 comprise a series oi coils connected in parallel cirperiod ai ter deenergithrough the conductors cuit relation so as to reduce the total effective inductance of the neld winding, thereby to permit a more rapid discharge of the residual energy in the field winding through the resistor |02. This method oi securing quick response of the generator excitation system is merely illustrative of one system which will provide quick excitation response. Any system that will reduce the ratio oi the inductance to the resistance of the circuit will increase the speed oi response oi.' the circuit and produce the desired quick discharge oi the field circuit.

The winding oi the relay IB is connected to bc energized through conductors Illt and H0 from one phase of the generator 2 and is so constructed as to maintain its back contact element Il? in open circuit position when approximately normal potential is being generated oy the generator 2. 'I'he contact element H2 controls a shunting circuit for the resistor 94 which extends from one side of the resistor 94 to the conductors 96 and lil, the contact element l l2 and conductors 92 and 83 to the other side of the resistor 94. The resistor 94 may be so adjusted as to provide normal excitation for the generator 2 when the contact element 88 is in closed circuit position and the contact element H2 is in open circuit position. When the contact element M2 is in closed circuit position, greater than normal energizing current for the ileld winding is will be provided to eiect a rapid build-up oi the excitation ot the generator 2.

The system comprising the discharge resistor i132., the back contact element lD of the relay 2li, and the connections providing a quick discharge circuit for the field windings 23 of the generator L and the front contact element It ofthe relay 20 and the relay i8 for effecting a rapid build-up ol the excitation of the generator 2, may be designated as a quick-response excitation system Ior the generator 2. It is to be understood that the specific quick-response excitation system described herein is merely illustrative o! one practical means for producing the desired functioning, and that other means may be employed to produce similar results.

In the operation of the system, when it is desired to start the motors Il and IB, assuming the elements oi the system are in the position indicated in the drawing. the contact element 6I may be moved to the left-hand position into ensagement with the contact element 10, thus energizing the relay 8 and causing it to complete the energizing circuit ior the winding oi the closing coil of the circuit breaker l. At the same time the energizing circuit for the relay im will be completed, causing the contact element 8B to move to closed circuit position. The contact elementl i l2 will remain in closed circuit position, thus providing a greater than normal ener gization of the iield winding 23 to insure a quick build-up of the excitation oi the generator 2 until the output potential of the generator 2 has reached such a 'value as to actuate the relay IB and move the contact element lill to open circuit position, thus restoring the energioation of the field Winding 23 to normal. By these operations, it will be seen that the motors it and It will be connected tc the generator simultaneously with the beginning of the excitation of the generator so that there will he no initial generator potential to cause the excessive transient toroues illustrated Fig. 2 and that thereafter the output potential oi the generator 2 will loc huilt up rapidly to provide quick starting of the motors I4 and I6. y

If now it is desired to reverse the direction of rotation of the motors I4 and Il; the contact element 68 may be moved to the right-hand position into engagement with the contact element l2. This will deenergize the relay l and energize therrelay III. After a predetermined interval, the front contact element 40 oi' the relay 8 will move to open circuit position to deenergize the closing winding of the relay 4, and the contact elements 44 and 46 will move to closed circuit position to complete the hereinbefore described energizing circuits for the closing coil winding oi the circuit breaker 6 and the winding of the relay 20. During the interval that the relay 20 has been de-v energized, its contact element will have closed the discharge circuit for the field winding 23, thus decreasing the excitation of the generator 2 and decreasing its output potential, thereby causing the contact element II2 to move to closed circuit position. Immediately upon the reenergization of the relay circuit, however, the discharge circuit of the field winding 23 will be opened by the contact element |00 and the energizing circuit for the field winding 23 will be closed by the contact element 88. The excitation of the generator 2 will thereafter be rapidly built up by greater than normal energization of the field winding 23 until its output potential is substantially normal, energizing the relay I8 which opens the shunting circuit for the resistor 94 by moving the contact element H2 to open circuit position, thereby restoring the eld winding 23 to normal energization. Thus, upon reversal of the control switch I2, the eld winding 23 of the generator 2 will be discharged. The

`generator 2 meanwhile will be left connected to the motors I4 and I6 so that the motors may assist in quickly reducing the generator excitation, thus supplementing the action of the discharge circuit for the eld winding 23. At the end of the delayed release interval of the relay 8, the circuit breaker 4 will be opened and the circuit breaker 6 closed to reverse the motors I4 andy I6, and simultaneously therewith the field winding 23l of the generator 2 will be quickly brought up to normal by the action of the relays 20 and I8 as hereinbefore described.

The control system will operate in a similar manner in reversing the direction of rotation of the motors I4 and I6 in the opposite direction.

As described hereinbefore, the system functions to simultaneously connect the motor for energization by the generator and connect the eld winding of the generator for energization. 'Ihe essential action in the connection of the motors, whether in starting or reversing, is that the motors be connected so as to be energized by the generator before the excitation of the generator has been built up to such a value as to produce excessive transient starting torque. Accordingly, the generator field winding may be energized slightly in advance of the connection of the motor to the generator by any suitable time delay means. This may be done by any suitable means for delaying the closure of thejcontact elements 44 and 48 after the closure of contact elements 46 and 50, respectively, or by any other suitable means.

Thus, it will be seen that we have provided a control system for limiting the transient torques incident to starting and reversing induction motors and which shall function to start and/or reverse the induction motors in a minimum time commensurate with the limited transient torque.

In compliance with the requirements ot the patent statutes, we have shown and described herein a preferred embodiment of our invention. It is to be understood, however. that the invention is not limited to the precise construction shown and described but is capable ot modincation by one skilled in the art, the embodiments herein shown being merely illustrative of the principles of our invention.

We claim as our `invention:

l. In a control system. a generator, a motor which is energized by the generator. means for reducing the ileld energization of the generator, means for thereafter substantially simultaneously reversing the energization of the motor'and applying a greater-than-normal energization to the field winding of the generator until the generator output potential reaches a predeterminedvalue, and means responsive to the generator output potential for providing normal energization of the generator field winding.

2. In a control system, in combination, an induction motor, a generator connected to enerf gize the motor and having field windings comprising a plurality of coils connected in parallel circuit relation, a discharge circuit for the field windings of said generator, manual means for opening the energizing circuit and closing the discharge circuit for the generator iield windings to thereby quickly decrease the energization oi' the eld windings, means responsive to the actuation of said manual means for reversing the connections from the generator to the motor a predetermined time after the operation of the iield discharge means, means responsive to the actuation of said manual means for reenergizing the generator field winding with a greater-thannormai energization substantially at the time the motor connections are reversed, and means for restoring normal energization to the generator field winding after a predetermined interval.

3. In a control system, in combination, an induction motor, a generator connected to energize the motor and having iield windings comprising a plurality of field coils connected in parallel circuit relation, a discharge circuit and an energizing circuit for the generator iield windings, a relay having front and back contacts controlling said energizing circuit and said discharge circuit, respectively, means for reversing the connections between said generator and the motor, a control device, means responsive to the operation of said control device for deenergizing said relay and for actuating said reversing means after a predetermined time, means responsive to the actuation 'of said reversing means and said control device for reenergizing said relay, and means responsive to the output potential of said generator for so modifying said generator field energizing circuit as to effect a greater-than-normal generator field energization while said relay is energized and the output potential of said generator is below a predetermined amount, whereby the motor will be reversed without excessive transient torque and in a minimum period of time.

4. In a control system, an induction motor, a generator for energizing the motor, a manually actuable quick response system for reducing the excitation and then increasing the excitation oi the generator in response to the initial manual actuation, means responsive to the initial manual actuation for disconnecting the motor from the generator and means responsive to the initial manual. actuation iol: timbre connecting the to `tile genere; n loi gization in a reverse direction before tine meow tion of the generator has increaeed to suoli; an amount as to cause excessive transient moin# torquew .5Ii ln a control syl-item, on ir lu i alternating current igiveratii il i motor, manually :letux ul the energlzatilon nl' tino nio sponsive to the actuiitiim oi' the roi/eremo mirano for reducing the field eneigization of the genero tor when said reversing means is actuated emi thereafter automatically inoreaeing the generator excitation to normal.

6. In a control system, the combination 4with un induction motor and an alternating current generator for energizing the rnotoii', of n'ianualiF operable means for reversing tile enei'gizotilon oi" the motor, means responsive to the actuation of said manually operable means for rednoing" tile eid Ienergirtaticin of the gencrotoiw and menno responsive to th otentinl oi" tine generator for providing a grof r than oorinni @nomination of the generator ileld winning' nnili the output potential of the generator lum; rooolfieol o determined value,

7. In a control system o. irioixoi', generator for energizing the motor, nian'ifinlly onei'iiiele lil moo fior reilniiziitw nonerotoi', molina resnomsive to inn mili manually operable means ope determined time after the actriitt.. mmmally oineroblo menne im" 1li 'tranieoiiely revorsii" time oneigliiotio nifl ow it e n imnntmlly operable met-mn """u mit, Flemming;

:nein Windiiig oi the gelmini/or, menno tresnonive to the actuation of solo marnix-lily opiierable means automatioally reversing the oooiiiections to tile motor st predoterminerl time titer iictnetio of sold monuaily operable menne, :n n sive to the nistnution oi' said monto moons oiitornoticeiiy i'eenergining ing" of the generator with n grantor conn energizotion .substantially oi; the

'to gonei'otor notei'itlol im" i; turning; tile @fieneioizoi :iielii onergiocitioi inni. mitei- 9. lo, leiterniiirieil genemtoi" y llos lneen estnlilioioeil.

'iS-lijn 

